Objects on a map are represented using a variety of symbols. There are point symbols which represent such things as buildings and trig stations. Line symbols are used to represent things like roads, railways and rivers. Area symbols are used to represent such things as vegetation, cultivation and the sea. The meaning of these symbols is contained in the key. It is important that you study the key very carefully whenever you are presented with a map. This will allow you to better understand the map. The picture below shows a key for a map of Basseterre, St. Christopher. It contains symbols one would find on a typical topographical map of the Caribbean. Study it carefully.
Most of the items in the key are pretty straightforward and therefore need no further explanation. However, on the right side of the key, the fourth item from the top reads “Contours V.I. 50”. This requires some explanation. A contour is a line which joins points of equal elevation (height above sea level) on a surface. On a map, they are usually brown. “V.I” stands for vertical interval, which is the difference in elevation between one contour line and the next. On this map the vertical interval is 50 units. Since the heights on this particular map are given in feet, V.I. 50 means that the difference in height (vertical interval) between one contour line and the one next to it is 50 feet.